Process for the reduction of zinc, cadmium, and mercury



Jan. 28, 1941. H. A. BLACKWELL ETAL 2,229,716

PROCESS FOR THE REDUCTION OF ZINC, CADMIUM, AND MERCURY Filed Nov, 15, 1938 Patented Jan. 28, 1941 UNITED STATES PATENT OFFICE PROCESS FOR. 'rIIE REDUCTION OF ZINC, CADMIUM, AND MERCURY Company Application November 15, 193s, seriai No. 240,564 In Great Britain January 11, 1938 2 Claims.

This invention is for a process for reducing the metals zinc, cadmium or mercury from ores or compounds containing one of these metals.

One of the objects of the invention is to pre- 5 pare any of the metals zinc, cadmium or mercury in a high state of purity by reduction from the appropriate ore in such a way that the metal distils from the reaction mixture without any fusion of the reaction mixture taking place, and this fnOn-fusion is advantageous to the yield obtained by the process. Another object of the invention is to provide a simple and effective prcess for reducing any of the metals zinc, cadmium or mercury by means of reducing agents which are abundantly available -at low prices and which, for the purpose of the process, do not require expensive grinding to permit intimate contact between the ore to be reduced and the reducing agent.

A specific object of the invention is to distil zinc metal directly from a reaction mixture composed of zinc ore or compound and a reducing agent and to burn the' distilled metal in air to obtain zinc white in a ne state of sub-division.

The invention ls basedon the discovery that a large and rapid yield of pure zinc, cadmium or mercury vapour can be obtained directly from a mixture of the -appropriate ore and a reducing agent by- (a) The employment of a means for heating the reaction mixture which permits the reaction chamber to be exhaustedto such a degree as will prevent reverse reaction between the vaporised metal and carbon monoxide;

(b) 'Ihe employment of a means for creating a rapid heat throughout the entire reaction mixture so that metallic vapour is given olf directly from the solid substances, that is to say metallic vapour sublimes from the reaction mix;

- (c) The employment of a reducing agent which remains solid at the temperature ofthe reaction and which is an electrical conductor so that high frequency induction heats the reducing agent which is dispersed throughout the reaction mix quickly and without liquefaction or sintering of the charge.

'Ihe preferred reducing agents are-graphitic carbon, silicon and silicon alloys such as ferro silicon. Silicon and silicon alloys always contain, in their commercial form, small percentages of free carbon; these small percentages of carbon during the reduction process are converted into carbon monoxide so that unless the pressure within the reaction chamber is reduced to such 5 a degree that reverse reaction between the carbon monoxide and the distilled metal is precluded then no satisfactory yields can be obtained.

One form of apparatus suitable for carrying out the `process will now be described with refer- 5 ence to the accompanying diagrammatic drawing which is a sectional elevation of a form of high frequency furnace and condenser collector for the distilled metal; the apparatus forms no part of this invention;

The high frequency furnace is a well known type comprising a steel container I-Il provided with an internal copper shield II and external stiffening bands I2. The coil is shown at I3 with terminal coil connections I4. The refractory furnace lining I5 is supported by insulated blocks I6 and Il land its external surface 'is cooled by water circulated through the coil I3 by the inlet and outlet conduits 36 andv 3l, one only being shown.

The collector-condenser I9 is in the form of an inverted dome and has a depending skirt 24 which extends close to the furnace lining to. direct the vapours towards the upper and cooler interior surface of the dome. The exterior of the dome is cooled by water flowing through a coil of copper piping 25 which may also serve as a high frequency coil for fusing the metal which is condensed on the dome. The lower interior part of the dome may have an internal channel 20 (shown in dotted lines) the outlet of which is controlled by a valve 2|.and this channel would serve to collect magnesium fused in situ on the dome and allowed to ow from the channel into ingot moulds. The crown of the dome is pro'- vided with a suction outlet 22 which is connected to an exhausting air pump 26.

The form of dome shown in full lines may be connected by water cooled dismountable joints 23 to the high frequency furnace. The purpose of this arrangement is to permit the dome with the metal condensed thereon to be removed for fusion of the condensed metal dust and afresh dome used for a new furnace charge. The cooling water may be allowed to ow on to the joints and collected in the trough 21 and be drained away therefrom to a sump. Where the apparatus is worked on discontinuously, that is to say the collector condenser I9 is removed after each charge and a new collector condenser of similar construction substituted for condensing a new charge, then the coil 25 will not be used for fusing the condensed charge in situ; this fusing operation will be separately performed.

The high frequency alternating current generator used for an induction furnace is a well known apparatus. The generator which s upplies )the coil I3 is indicated generally at 28, and as is well known comprises the usual thermionic valve generator. The mains are indicated at 29 and the leads 30 from the generator are connected to the terminals I4 for the coil I3. A similar form of generator may be connected to the coil 25 and this generator is indicated at 32, the leads 33 supplying the necessary energy to the coil 25.

In using the above described apparatus zinc sulphide is broken up and intimately mixed with a basic material, and reducing agent for example coke, and charged into the space surrounded by the lining I 5 or into the crucible 38; the condenser portion I9 is adjusted and sealed by suitable means such as bolts 39 and nuts 40 and then, with the use of the air pump, the atmosphere Within the reaction chamber is evacuated of air and gas to a degree sufcient to prevent reverse reaction between the vaporised zinc metal and carbon monoxide. The high frequency valve is tuned in to give the desired frequency of current which is of the order of one million cycles per second and almost immediately the coke is increased in temperature. The heat generated in the' coke is localised within the reaction mix and since the coke is in intimate contact with the ore to be reduced a high thermal eiciency is obtained. The gases initially resulting from the application of heat (when the coke is raised to a red heat) are quickly eliminated by the suction pump and the temperature of the charge is raised to above the boiling point of the metal being distilled having regard to the reduced pressure Within the reaction chamber. The vaporised metal rises into contact with the cold surface of the collector condenser I9 and is condensed on the inside surface thereof, from there the distillate can be removed by fusion and cast into ingot moulds. Preferably the barometric reading of the atmosphere within the reaction chamber is not allowed to rise substantially above 2 millimetres of mercury otherwise the metallic vapour will not condense but will reduce carbon from the carbon monoxide insteadv and if the vapour density of the mixed vapours is insufciently reduced then reverse reaction will occur.

To illustrate the process one example of the preparation of zinc from zinc sulphide will be described. The zinc sulphide is broken up and intimately mixed with a basic material such as lime and with sufiicient coke or graphite to complete the reduction. The coke is added in two portions, 66% of which is crushed and intimately mixed with the charge and 34% of which is added in the form of lumps approximately 11/2 in diameter, the reaction takes place as follows:

The reaction mixture is placed in the closed furnace and the pressure within the furnace is reduced to about 2 millimetres of mercury. The high frequency furnace is started up and the coke heated to a. temperature above 930 C. At this temperature the sulphide is reduced and zinc metal sublimates from the reaction mix without fusion of the mix. The carbon monoxide generated is carried away from the chamber by means of the evacuating pump and the metallic zinc will condense on any surface which is cooled to a temperature below 930 C. over which the vapour passes.

If suitable arrangements are made for burning the Zinc vapour in air it is converted into zinc oxide which can be deposited as a powder in a ne state of division and collected for use.

For the preparation of mercury the ore. known as calomel which consists chiefly of chloride of mercury is mixed with lime and sufficient coke to produce a charge which is chemically balanced for the reduction process; that is, the lime is sufficient to combine with the chlorine of the ore and the carbon is sufficient to combine with the oxide of the lime. The carrying out of the process is as described in connection with the reduction of zinc.

For the preparation of cadmium, yellow sulphide of cadmium is mixed with sufficient manganese oxide to form manganese sulphide and sufficient coke to combine with the oxide of the manganese. The recovery of the metal is as described with respect to the zinc metal.

As a reducing agent commercial ferro silicon may be employed as an alternative to coke, the preparation of the reducing agent being the same as hereinbefore described with regard to coke.

While a degree of vacuum within the reaction chamber considerably greater than 2 millimetres mercury may be employed if desired, the vacuum should not be suficiently high to cause a glow discharge to be generated in the reaction chamber, which discharge will absorb the energy of the induction coil. Any of the well known expedients for avoiding such glow discharge may be used when the process is carried out under very high vacuum.

We claim:

1. A process for the reduction of a metal of the group consisting of zinc, cadmium and mercury which comprises mixing an ore of the said metal in finely divided form with the usualreduction ingredients comprising a reducing agent; the latter being graphitic carbon, at least part of which is in finely divided form; heating the reaction mixture by high frequency current of the order one million cycles per second at a pressure maintained below about two millimetres of mercury.

2. A process for the` reduction of a metal of the group consisting of zinc, cadmium and mercury which comprises mixing an ore of the metal in finely divided form with the usual reduction ingredients comprising a reduction agent containing an appreciable quantity of carbon, at least part of the said reduction agent being in finely divided form, heating the reduction mixture by high frequency current of the order of 1,000,000 cycles per second at a pressure maintained below the pressure at which reverse reaction between the vaporized metal and the gases present takes place.

HAROLD ALEXANDRE BLACKWELL. WILLIAM LAWRENCE TURNER. 

